 window.onload=function(){
            var a1=document.getElementById('a1');
            var a2=document.getElementById('a2');
            a1.onclick=shield;
            a2.onclick=cancel_shield;

            function shield(e){
                e.preventDefault();
                var shade = document.getElementById("shade");
                shade.style.display = "block";

                var modal = document.getElementById("modal");
                modal.style.display = "block";
            }

            function cancel_shield(e){
                e.preventDefault();
                var shade = document.getElementById("shade");
                shade.style.display = "none";

                var modal = document.getElementById("modal");
                modal.style.display = "none";
            }
        };